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> > Predicting a hatch

U2illApril 2nd, 2008, 12:13 pm
Pullman, Washington

Posts: 3

I first posted this in the mayfly section, but think this forum (more general) is a better place for it - so please forgive me for cross-posting.

Is it possible to predict a hatch (whether mayfly, caddis, stonefly, etc) based off of water, air, and weather data? So if I know streamflow, water temp, air temp, wind, cloudiness, etc...and I know what insects are on a that enough data to make a prediction like "There would be a good likelihood of a caddis hatch this afternoon"?
TaxonApril 2nd, 2008, 2:32 pm
Site Editor
Plano, TX

Posts: 1339

When someone asks an "is it possible" question, it is a bit risky to respond in the negative. Having said that, in my opinion, even if one knew all the factors you mention, unless one also knew that a particular insect emergence had occurred the prior day on that water, accurately predicting that it would occur again today would be unlikely.

Of course, there are some exceptions which come to mind. One would be BWOs, which can be reasonably weather dependent at certain times of year. Another would be those emergences which tend to progress up a stream or river at a fairly predictable pace.
Best regards,
Roger Rohrbeck
KinzuaApril 2nd, 2008, 4:31 pm

Posts: 20
Can one predict mayfly hatches based on the seasonal blooming of certain plants? I have heard when the dogwoods are blooming, the Hendricksons should be hatching. I vaguely remember a book some years ago that was based on this concept.

WbranchApril 4th, 2008, 8:30 am
York & Starlight PA

Posts: 2727
Concerning Baetis emergence over the course of decades I've seldom seen respectable Baetis hatches on bright sunny days. I have seen a great hatch on drizzly rainy days and then if the next day is bright the bugs don't show but if the next day is raining or overcast they will be back.
Catskill fly fisher for fifty-five years.
GONZOApril 5th, 2008, 10:47 am
Site Editor
"Bear Swamp," PA

Posts: 1681
Hi gang. Because I've finally found a few free moments, I couldn't resist chirping up on this thread. (Yesterday, I presented a paper at the Fly Fisher's Club of Harrisburg that touched on this, so the topic has been on my mind.)

William (U2ill), with regard to your "Is it possible...." question, my answers would be "yes" and "no." The negative response is based on the factors you list in your post. Some of these have limited relevance, and the others are inadequate. You seem to be looking for a remote way of predicting a hatch with a high degree of accuracy, and I don't know of any. (At least, short of having a friend on the stream that can employ the method I'll describe.)

The affirmative response is that many hatches can be predicted very accurately, but it usually requires more of a "boots on the ground" (OK, in the water) approach. The general method involves regular sampling of the larval populations in order to gauge their maturation (to predict the overall emergence period) and having some idea of the optimal water temperature for a particular hatch (to target the daily emergence). This method is quite easy when sampling mayfly nymphs that inhabit rather fast water (by using a hand seine, or simply by turning over a sufficient number of rocks) and somewhat more complicated for slow-water species or burrowers. The same approach can often be used for many stonefly and caddisfly species, but reading the signs of maturation in the larvae can be a little to a lot more complicated.

John (Kinzua), making the connection between flowering plants and aquatic insect hatches is fun, but that connection is rather broad and inexact, in my experience. For example, over the years I have noticed a loose correlation between the blooming of wild flox and the hatching of what I call the olive morning dun (Drunella lata, formerly cornuta). This may be related to similar "degree-day" requirements (see that discussion in the "Water temperature effect" thread). But I think that there are factors that might influence land plants and water animals differently. This method can be entertaining in a "woodsy" sort of way, but I doubt that it is really reliable.

Best to all,
FalsiflyApril 5th, 2008, 1:00 pm
Hayward, WI.

Posts: 660

I couldn't resist chirping up on this thread.

Its good to hear from you, I was begining to wonder. And by all means please continue to chirp.
When asked what I just caught that monster on I showed him. He put on his magnifiers and said, "I can't believe they can see that."
GONZOApril 5th, 2008, 6:56 pm
Site Editor
"Bear Swamp," PA

Posts: 1681
Hi Falsifly--I'll try to rub my wings together more frequently. :)
TroutnutApril 5th, 2008, 9:08 pm
Bellevue, WA

Posts: 2727
Great answer as always by Gonzo -- nice to have you back online! It's so much easier to answer questions when the answer is, "What Gonzo said..."

I do have one thing to add, though.

is that enough data to make a prediction like "There would be a good likelihood of a caddis hatch this afternoon"?

You definitely can't make predictions on the level of a big taxonomic category like "a caddis hatch." There are caddisflies that hatch under all different kinds of conditions. Predictions have to be made on the level of species, or closely related groups of species. Some seem to have an affinity for cloudy days, some don't like it windy, etc.

It is possible to make some fair predictions about hatch quality based on the weather, if you know how the recent weather has compared to normal, and you know which bugs would normally be hatching and what kind of weather they prefer.

For example, in the early spring, when many bugs are waiting for the weather to get warm, you might have a cold front for a week, followed by a really warm day... and you could expect good hatches on that really warm day.

I suspect the reverse is also true. In the heat of summer, when bugs are mostly emerging at night when it's cooler, an unusually cool or cloudy day might trigger good hatches.

One of the best ways to judge hatches, when you know the region and the usual hatch schedule, is by looking at what else has been hatching recently. Hatches usually follow a relatively predictable sequence on any given stream, and the whole sequence shifts with the weather, so if Species A usually comes about 5 days after Species B on a particular river, it will probably continue to do that regardless of whether it's a warm year or a cold year.

These are kind of vague rules of thumb... but that's as good as hatch forecasting gets.
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist
GONZOApril 6th, 2008, 12:01 pm
Site Editor
"Bear Swamp," PA

Posts: 1681
As usual, Jason's comments have a way of drawing me further into the discussion. In this case, I'm wondering whether I might have misinterpreted what U2ill was really asking.

Let's assume that we are planning a trip to a distant stream and can't employ the method I described above. Here's what I'd do:

First, start with a sense of how important a hatch-matching strategy might be on the stream or stream section to be fished. For example, if I am planning to fish a small hard-rock headwater, it rarely matters very much. In contrast, if the stream is larger and richer, matching the hatch can be much more important. The time of year, time of day, weather, and level of pressure can also factor into this consideration.

Second, gather the best information you can find about important hatches on that stream at that time of year. A general knowledge of the hatching calendar, personal records, hatch charts, online information, and local advice can all be used to whittle down the possibilities. Just beware of vague catchall references like "tan caddis" or "BWOs" that can include nearly as many possibilities as they rule out.

Third, adjust for long-term and short-term temperature trends. Generally, cooler than normal long-term trends will retard the calendar, and warmer than normal will advance the calendar. Cool short-term trends will tend to push hatches closer to the middle of the day, and warm short-term trends will push hatches toward either end of the day (morning and evening).

Fourth, adjust for water temperature in the stretch you intend to fish. On most freestone streams, hatching in the lower (usually warmer) waters will come earlier in the period, and hatching in the upper (usually colder) waters will be later. Waters with more stable temperatures, like spring creeks and some limestone or tailwater streams, will not have quite as much variation, but beware of tailwaters with highly variable cold-water releases. The upper Delaware is a good example--hatches can be scattered all over the calendar by releases.

This basic approach can be very effective, but I'd be lying if I said that it was surefire.
WiflyfisherApril 7th, 2008, 5:06 am

Posts: 649
To add to this...

Check the backwaters and spider webs along the banks for dead spinners and duns trapped in webs or drowned. Also, stonefly nymphs, dragonfly nymphs and some mayfly nymphs climb out of the water to emerge. You can generally find their shucked cases along the banks or protruding objects.

Mayflies nymphs ready to pop are generally very active on the bottom and have very dark wing cases too.
John S.
MartinlfApril 7th, 2008, 9:14 pm
Palmyra PA

Posts: 3176
Good tips, as usual. I've been monitoring olive hatches this year, and with the help of several guys have come up with the following. Weather may not exhibit a clear pattern. Temps in PA have been all over the place, and so has the level of cloud cover. But on sunny days the hatch has generally seemed to start later in the day and either be more sparse, or fish have been less inclined to rise. Cloudy days have seen the hatch start hours earlier, and fish have been more active. All things being equal, and they haven't often been, the later in the season the later in the day the hatch comes off. I'd be curious to hear others' olive observations, and the states they come from.
"He spread them a yard and a half. 'And every one that got away is this big.'"

--Fred Chappell
KinzuaApril 8th, 2008, 8:16 pm

Posts: 20
I've been chasing the early season olives for the last 4 years on Spring and tend to agree with your observations. This year they seem to be late and the hatching short-lived, but I'm not on the stream enough to say for sure. It's neat when suddenly start popping and the fish turn on. What is perplexing is as the hatch wanes, most of the trout seem to shut down completely and will ignore the last of duns that trickle over their feeding lanes. I suspect the fishing pressure may have something to do with this.

MartinlfApril 8th, 2008, 8:26 pm
Palmyra PA

Posts: 3176
I'm sure you're right. But at least we have sulphurs coming to wake them up again, and Grannoms and blue quills before that on a few streams.
"He spread them a yard and a half. 'And every one that got away is this big.'"

--Fred Chappell
WiflyfisherApril 9th, 2008, 3:48 am

Posts: 649
What is perplexing is as the hatch wanes, most of the trout seem to shut down completely and will ignore the last of duns that trickle over their feeding lanes.

It may be as the hatch dwindles the trout turn to some other morsel that is more readily available near the stream bed. This time of year the river bottom is alive with a smorgasbord of nymphs and larva that may capture their attention and waste less energy. The tough part is figuring out what it might be.
John S.
MartinlfApril 9th, 2008, 9:29 am
Palmyra PA

Posts: 3176
Yes! Shawn recently asked me why I was fishing a Walt's Worm in the middst of olive season, and why it had worked. (Now that I think of it he may have been casting aspersions on my simple and ugly fly--something he's not above.) This answers his question nicely, for fact of the profusion of food on lower levels not only argues why fish may turn to the bottom as a hatch wanes, but also why some may be dining there even as the hatch waxes.
"He spread them a yard and a half. 'And every one that got away is this big.'"

--Fred Chappell
GONZOApril 9th, 2008, 3:16 pm
Site Editor
"Bear Swamp," PA

Posts: 1681
Hmmm...I apologize if I seem to be quibbling, but I'm not sure I can accept the above explanation as quickly as Louis did. Yes, there may be a "smorgasbord" of nymphs and larvae at this time of year, but the explanation also seems to suggest that those benthic invertebrates are constantly available to the trout and easy to capture. Surely, the early Spring underwater smorgasbord also includes some relatively large and meaty items (at least, compared to the little baetids). If these items are readily available for easy capture, it seems to beg the question of why the trout would rise to the little baetids in the first place. Drift-feeding trout see broad and frequent fluctuations in available prey, on the surface and below. The type of drift that is termed "constant" (also known as accidental) is usually characterized by very low numbers of prey.

Instead, I'd suggest that trout are drawn to the surface during a hatch by the increasing number of nymphs moving in that direction to emerge. When sufficient numbers of nymphs/emergers/duns accumulate at or near the surface, trout rise frequently. Some may even hold just under the surface for a while during the temporary bonanza. But being at or near the surface is a behavior with inherent risks. When the number of prey available near the surface decreases, so does the incentive to take the risk of rising--especially to a few small duns. (Unless, I suppose, the trout is very young or very hungry.)

I also think that Kinzua's original supposition about fishing pressure could also be a factor--especially on a stream like Spring. I often find that trout are more easily "put down" by fishing activity when numbers of emerging insects are low. I'd guess that there is a similar risk/reward aspect to this.

WiflyfisherApril 9th, 2008, 4:03 pm

Posts: 649
Ah, the fun of quibbling... :)

When the numbers of prey available near the surface decrease, so does the incentive to take the risk of rising--especially to a few small duns. (Unless, I suppose, the trout is very young or very hungry.)

I remember on the Henry's Fork last summer clearly watching several 22"+ rainbows coming to the surface to feed on PMD emergers drifting by. They were infrequent and tiny morsels to say the least, but the fish were not young and they looked well fed.

My point was instead of all trout feeding totally turned off, a change of menu might be just the thing they will engulf.

Nice to see you on the forum Gonzo. I hope all is well for you!
John S.
GONZOApril 9th, 2008, 5:13 pm
Site Editor
"Bear Swamp," PA

Posts: 1681
Fair enough, John, and thanks. The exceptions and qualifications you mention are very well taken. I was just hoping to suggest that drift feeding at all levels is subject to many variables, despite the seasonal abundance of prey on the streambed. The growing body of scientific literature about this can be worth investigating; though it can also be rather stiff going, so I'd suggest taking it with equal doses of stiff drink. :)

PS--Because Kinzua described his tying as 99% dry fly, I'm not sure any of these ideas are of much help. ;)
GONZOApril 9th, 2008, 7:13 pm
Site Editor
"Bear Swamp," PA

Posts: 1681
Hey John, just to push the fun of quibbling a bit more, here's an afterthought I had about the Henry's Fork example you cited: If the fish were feeding infrequently on PMD emergers, then it might just be that they were drawn toward the surface by an accumulation of the nymphs and were taking the majority of their food subsurface. This would be different than the instance of a trickle of lingering duns at the end of a hatch that Kinzua describes, and wouldn't really be an exception to what I was suggesting. I'd also add that trout in big waters like the Henry's Fork can be very picky, but often are not as nervous and spooky as trout in smaller waters like Spring, even under fishing pressure. Just some additional thoughts and conjecture. What do you think?
Shawnny3April 9th, 2008, 7:30 pm
Pleasant Gap, PA

Posts: 1197
(Now that I think of it he may have been casting aspersions on my simple and ugly fly--something he's not above.)

Me?! Aspersions?! Louis knows full well I have trouble enough casting dry flies, let alone SAT words. He should be ashamed at the, umm, aspersion.

This has been one of the better discussions we've had in awhile, coinciding almost exactly with Gonzo's return. Hmm...

I have my own opinions about these matters, but they're based on so little actual fact that I'll spare the rest of you my musings. Now if only I could get those voices out of my own head...

I will say that I fished Louis's Worm (a Walt's Worm about four times the size of any worm Walt ever fished, otherwise known as the Tan Dog Chow Deluxe) the other day for a few hours during the baetis activity and only managed to foul-hook one fish with it, dead in the eyeball. Next time I plan on making the worm bigger, with more weight and on a treble hook, to improve my odds.

Finally, this year I've started doing something I've rarely done before, and I hope I have the discipline to make it a ritual. Before fishing, I'll take my seine and hold it in the water without kicking around any stones. I've been amazed at how many insects I've captured that way, including both nymphs and duns. That way I not only get a feel for what is inhabiting the stream but what is actually drifting down the stream (I use italics not for emphasis but to show that I'm cool and know how to do it). This has helped me identify hatches the moment I get to the stream and helps me match the colors and sizes (of the nymphs, in particular) better than I'd be able to do by waving my arms at passing duns.

Last time out I also hooked a piece of bark at one point, and it literally had hundreds of baetis nymphs clinging to it. Some had black wingcases, some not, and there were a variety of different colors represented from tan to dark brown to olive - it was pretty cool to compare them.


P.S. As a totally unrelated aside, my 4-year-old came home from pre-school yesterday and told me that a firefly has "six legs, a head, a thorax, and an abdomen." When asked what that meant a firefly was, he replied, "a bug."
Jewelry-Quality Artistic Salmon Flies, by Shawn Davis

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